Common mode noise propagation between primary and electrically-isolated secondary circuits in Switch Mode Power Supplies (SMPSs) has long been a problem. High frequency noise, or Electro-Magnetic Interference (EMI), generated by the switching transistor(s) in the primary circuit and the rectifier(s) in the secondary circuit are conducted, via primary-secondary stray capacitances, either back to the line supplying the SMPS, or into the load that it is powering. Such noise is also radiated and may adversely affect any sensitive nearby components and circuits. Strict conducted and radiated emissions standards must be complied with in marketed devices.
Stray capacitances between the primary-side and secondary-side circuits are predominantly associated with the isolating power transformer(s) but may also arise from other mechanisms. Examples include the structure of the switching power transistor that often gives a large area of radiating conductor which can couple to secondary-side conductors, and PCB conductor traces.
Methods to reduce interference generated by isolating switch mode power supplies, which apply to common-mode noise arising from capacitive coupling associated with the transformer and also to structures elsewhere in the power supply (e.g. electrodes of the power switch and PCB conductors), divide into three broad groups:                Y-capacitors between primary-side and secondary-side to bypass the noise;        Shields between primary-side and secondary-side circuits;        Cancellation of residual signal.        
Y-capacitors are effective but have technical shortcomings such as earth leakage current and behaviour in surge conditions. Hence there is a need for other low-cost techniques to deal with the residual noise.
Electrostatic shields provide a partial solution to the passage of noise via capacitive displacement currents through the stray capacitance coupling in the transformer. A shield is typically composed of either an incomplete turn of foil or a bobbin-width wire winding having a small number of turns, placed between primary and secondary windings. Common mode noise then couples across the winding-shield stray capacitance and returns to the circuit connected to the shield. However, even if multiple shields are connected to primary and secondary circuits, some residual noise signal remains.
Cancellation of residual noise signal has been attempted in various ways, with varying levels of success. For example a passive common mode noise reduction circuit is described in WO 03/098788. The previous attempts fall into the following categories:    1. Additional internal transformer structures. Examples include:            a) Reducing a potential difference between primary and secondary shield windings (see, for example, JP 1995045451 Hitachi).        d) Cancellation and balancing windings attached to the input and output windings, respectively (see, for example, U.S. Pat. No. 6,549,431 Power Integrations). Such additional structures typically add cost and bulk to the transformer, and may degrade other performance factors such as leakage inductance. Furthermore, optimisation of such structures for effective cancellation is often not straightforward.            2. Adaptations to transformer structures, such as matching voltage gradients in adjacent primary and secondary coil layers (see, for example, U.S. Pat. No. 5,107,411 Philips). These techniques tend to offer only modest improvements in noise rejection, and are difficult to optimise.    3. Partial coupling of inverse phase noise signals to the secondary winding:            a) Using an amplifier connected to both primary and secondary circuits to generate a counteracting noise voltage, which cancels the residual noise voltage (see, for example, U.S. Pat. No. 6,879,500 University of Hong Kong).        b) Using an auxiliary winding in opposite phase to the primary winding, and cancelling the residual noise voltage by driving a primary-secondary shield foil with this opposing phase auxiliary voltage, with a capacitor between the auxiliary winding and the shield (see, for example, U.S. Pat. No. 5,724,236 Motorola).        c) Using an auxiliary winding in opposite phase to the primary winding, and using its signal to cancel the residual noise voltage by way of external components (U.S. Pat. No. 6,879,500).        d) Using an auxiliary winding in opposite phase to the primary winding, with a ‘Y-capacitor’ between auxiliary and secondary circuits to inject an inverse phase noise signal directly into the secondary circuit (see, for example, U.S. Pat. No. 4,625,270 AT&T, and JP59129571 TDK).        
In the approach taken in 3.b) above, as described in U.S. Pat. No. 5,724,236 (ibid), a cancellation signal can be applied to a main screen between primary and secondary windings. However this increases the impedance from the shield to the signal reference voltage (typically one or other pole of the high voltage supply). This is undesirable because it increases the impedance of the shield to the main noise current couping onto the shield (typically from the primary) and hence allows a small noise voltage to be present on the shield. This couples to the secondary via the mutual capacitance, so the shielding effectiveness is impaired.